The present invention relates to an improved mounting arrangement for angle measuring devices of the type which include a graduated disc, a connecting hub mounted to the disc and secured to the drive shaft of an external drive unit, a stator mounted to an attachment surface of the external drive unit, and a scanning unit mounted to the stator to scan the disc.
In angle measuring devices of the type described above, the graduation disc is mounted without a bearing of its own. Rather, the graduation disc is mounted to an element, such as the drive shaft of a motor, for example, which is provided with its own external bearings. Another prior art approach is to provide the angle measuring device with a journaled shaft of its own which includes bearings for guiding rotary movement of the graduated disc with respect to the stator. In comparison the first system described above which contemplates the direct fastening of the stator to the drive unit, this second approach requires an additional bearing, an additional shaft, and some means for coupling the angle measuring device shaft to the external shaft. Thus, the direct fastening of the stator to the drive unit provides a particularly simple and economical construction. Furthermore, by directly fastening the stator to the drive unit and the graduated disc to the drive shaft, any possible twisting of the coupling between the graduated disc and the drive shaft as a result of angular acceleration of the drive shaft can be eliminated, along with frictional torques associated with graduated disc bearings. Finally, the total length of an angle measuring device which is directly mounted to the drive unit can be substantially reduced as compared to an angle measuring device in which a separate bearing shaft is provided for the graduated disc.
A disadvantage of prior art angle measuring systems in which the stator is directly mounted to the drive unit relates to the precision with which the angle measuring unit must be aligned. Since the graduated disc is mounted to the drive shaft of the drive unit and the stator of the angle measuring device is mounted to an attachment surface of the drive unit, the graduated disc must be precisely oriented with respect to the stator during the installation procedure. Thus, it has often proven to be necessary to perform precise adjusting operations during the mounting of such angle measuring devices. Depending upon the precision of the angular graduation used on the graduated disc, expensive calibration devices, qualified personnel, and significant time delays are often required to mount such angle measuring devices properly in place.
There is known to the prior art an angle measuring device which incorporates a two-part stator. This prior art device is installed by mounting first the lower part of the stator to the drive unit. For example, this lower part may serve to mount the illuminating arrangement and the scanning plate of the angle measuring system. This lower part is positioned precisely with respect to the external drive shaft of the drive unit by a calibration device. Then the graduation plate is positioned on the external drive shaft and clamped in place via its connecting hub. Finally, the upper part of the stator, which includes photosensors and pulse forming circuitry, is installed on the lower part of the stator and a casing cap which includes electrical conductors is mounted in place. In this prior art device, the photosensors mounted on the upper part of the stator must be arranged in place precisely with respect to the lower part of the stator. For this reason, an adjusting operation is required in order to mount this angle measuring device in place, or alternately, increased expenditures for fitting pins or similar arrangements must be accepted. Furthermore, the electrical conductors between the upper and lower parts of the stator must either be arranged to be flexible or be provided with connectors. Both of these approaches can lead to measuring errors.
Another prior art device utilizes a stator which is sickle-shaped. In mounting this second prior art device in place to an external drive unit, the graduated disc is first mounted to the external drive shaft via a connecting hub, and then the complete stator is placed on the attachment surface of the external drive unit and shifted transversely to the drive shaft until the scanning unit is oriented properly with respect to the graduated disc. In mounting this second prior art device in place, an adjusting operation is required to align the scanning unit properly with respect to the external drive shaft and the connecting hub, both in radial and tangential directions.